U.S. Pat. Nos. 4,905,410 and 5,485,698 disclose tree treatment capsules including a plug for insertion into an end opening of an internal cavity of a container. The capsule is introduced into a bore formed in a tree, with a protruding end of the plug engaging a bottom of the bore. An impact force is applied to a head element of the plastic container to move the head element towards the plug and break a shank of the container. Chemical agent is released from the cavity when the shank is broken in this way. Generally, the use of these known capsules has advantageous effects and provides good final results, but involves rather time-consuming preparation due not only to the large number of bores required but also to the comparatively strict requirement for appropriate dimensions, especially depth, of the bores formed in the porous elements. EP2066171B1 discloses a capsule providing a container and a plug for insertion into an open rear end of the capsule and for closing the open end to retain treatment substance in a cavity. At the open end the capsule has a stop flange, for abutting an outer wall of a tree etc. when inserted into said bore. At the outer end this capsule has a straight cylindrical wall adjoining the stop flange. Generally, with all prior art solutions within this field it is a matter of great concern to maintain the capsule leak proof outwardly, towards the open end.